This renewal continues research on estrogens (E) and progestins (P) in luminal, estrogen (ER) and progesterone (PR) receptor-positive human breast cancer. In the preceding cycle 27 papers were published. Relevant to this renewal, we: 1. Developed fluorescent mouse models of ER+PR+ tumors that metastasize to lymph nodes (LN). Compared to primary tumors, LN metastases are E resistant. 2. Demonstrated that ER+PR+ tumors contain a minor ER-PR- subpopulation that expresses cytokeratin 5 (CK5). These rare cells may be tumor-initiating and are expanded by P. 3. Initial ER+PR+ models of distant metastasis show hormonal influences on organ-specific engraftment. Metastatic breast cancer kills more than 40,000 American women each year and two-thirds of these tumors retain ER or PR. Despite clinical evidence that ER+PR+ tumors metastasize, the role of women's steroid hormones or their receptors on metastases is unknown, due to lack of models. Hypotheses: 1. E and P play critical roles on LN and distant metastasis of ER+PR+ disease. 2. In ER+PR+ disease, an ER-PR-CK5+ subpopulation with tumor-initiator properties is expanded by P and influences metastasis, dormancy and drug resistance. 3. Luminal ER+PR+ breast cancers exhibit receptor plasticity characterized by receptor loss, in a process driven by P. AIM 1. Metastasis Models of ER+PR+, hormone dependent breast cancer and role of estrogens. To develop models of ER+PR+ metastasis starting from solid tumors or circulating tumor cells, and test the hypothesis that E and P play a role in metastatic engraftment of ER+PR+ disease. AIM 2. Tumor-initiating cells in ER+PR+ breast cancer, tumor dormancy and drug resistance. To test the hypothesis that ER+PR+CK5- breast cancers harbor rare pre-existing ER-PR-CK5+ cells with tumor-initiating properties. ER-PR-CK5+ cells are expanded by P. We engineer models to study constitutive and P-regulated live ER-PR-CK5+ tumor cells and their role in recurrent disease. AIM 3. Towards a new biology for progesterone in luminal breast cancer. To explore a novel view of P in ER+PR+ breast cancers focused on P regulation of tumor-cell phenotype. We test the hypothesis that luminal breast cancers exposed t exhibit receptor plasticity associated with receptor loss. We study cell- biological and molecular mechanisms of this plasticity, develop methods to distinguish among putative ER-PR- cell subtypes, determine whether or not they represent a continuum of the same cells, and study CK5 regulation by P. In sum, the majority of breast cancer metastases are ER+PR+, so E and P must play critical roles in this process. E is the proliferative hormone. P is not. Rather, P partially extinguishes receptor expression. Receptor loss in a subset of ER+PR+ tumor cells is dangerous because these cells acquire tumor-initiating properties that secondarily promote tumor expansion. This has consequences on metastasis and disease recurrence.